1. Field of the Invention
The present invention relates to a circuit board cooling apparatus disposed in a flow soldering apparatus to solder electronic parts or the like on a circuit board and used for cooling the circuit board on which the electronic parts have been soldered.
2. Description of the Related Art
Up to now, in a flow soldering apparatus to solder electronic parts or the like on a circuit board, molten solder is supplied to a joint between the electronic parts to be soldered and the circuit board, the electronic parts are soldered onto the circuit board, and thereafter a circuit board cooling apparatus disposed in the flow soldering apparatus is actuated to cool a temperature of a soldering face of the circuit board down to a room temperature.
As a cooling method, for example, a heat radiation system by means of a cooling fan or a cooling fin is used. The former heat radiation system by means of a cooling fan answers a purpose of rapidly releasing the electronic parts from a heat stress suffered at the time of soldering, in which as shown in FIG. 10, air is blown by a cooling fan 4 against a soldered portion 3 on a soldering face 2 of a circuit board 1 at an exit of a solder bath (not shown) to supplement the effect of natural cooling. Chilly air (15 to 20xc2x0 C.) may be auxiliarily blown to speed up a cooling rate. The latter heat radiation system by means of a cooling fin also answers a purpose of rapidly releasing the electronic parts from a heat stress suffered at the time of soldering, in which as shown in FIGS. 11A and 11B, a cooling fin 5 of multiple structure as a radiation plate is disposed close to the soldered portion 3 on the circuit board 1 at an exit of a solder bath (not shown) to absorb radiation heat from the circuit board 1 which is warm immediately after soldering, thereby indirectly cooling the circuit board 1. Also, as another cooling method, there has been proposed a refrigerant contact system, in which a coolant 6 is brought in direct contact with the soldered portion 3 on the circuit board 1, as shown in FIG. 12.
The above-described conventional cooling apparatuses have the purpose of rapidly releasing the electronic parts from a heat stress suffered at the time of soldering, and, depending on cooling conditions after soldering, may invite such problems as lift-off, shrinkage cavities that make filet surface rough, faded gloss of the solder after soldering and deterioration in mechanical strength. And the prior art has not provided improvement to solve the above problems.
In general, when the soldered portion is rapidly cooled, its organization is closely packed thereby improving mechanical strength. It is supposedly desirable that the rapid cooling of the soldered portion is conducted at a cooling rate of 50xc2x0 C./sec or higher immediately after soldering.
Also, from the viewpoint of the environmental protection, which is promoted recently, Pb-free solder is increasingly replacing Snxe2x80x94Pb eutectic solder. which has been conventionally employed. Because the Pb-free solder is different in composition from the Snxe2x80x94Pb eutectic solder and has a melting temperature higher by about 40xc2x0 C., it is becoming increasingly important to increase the cooling rate for providing a high quality soldering.
In the above-mentioned prior art (FIG. 10), the cooling rate is about 1 to 3xc2x0C./sec in case of only blowing air, in which air is blown entirely over the soldering face 2 of the circuit board 1 and therefore the cooling capacity is decentralized making it difficult to set the cooling rate to 50xc2x0 C./sec or higher, which is not good enough to conduct an excellent soldering. In other words, the cooling fan 4 shown in FIG. 10 causes a turbulent air flow making it difficult to focus and target air onto a soldered joint surface for cooling. In addition, the cooling fan 4 cools also a portion to be soldered and a surface of solder in the solder bath which is located close to the soldered portion, as a result of which the quality of joint may be deteriorated.
Also, in a cooling method using a cooling apparatus shown in FIGS. 11A and 11B which is so designed as to absorb heat, the cooling effect is relatively small, and in case of a continuous operation the cool fin 5 cannot keep absorbing heat completely due to its limited radiation capacity and the cooling effect is further deteriorated.
Further, a cooling method shown in FIG. 12 provides a rapid cooling effect but may lead to such problems that a coolant 6 as a refrigerant accidentally gets into the solder bath which contains molten solder of a high temperature, and that mounted parts are rapidly cooled and destroyed due to a heat stress.
The present invention has been made under the above-mentioned circumstances, and therefore an object of the present invention is to provide a circuit board cooling apparatus which is capable of cooling a circuit board pinpointing a soldered portion, thereby improving a cooling rate.
In order to achieve the above object, according to a first aspect of the present invention, there is provided a circuit board cooling apparatus including a cooling compartment having an open top and a bottom with a plurality of openings for taking in air, in which a circuit board to which electronic parts are soldered is conveyed over the open top of the cooling compartment, and a soldered portion on the circuit board is cooled by cooling air taken in through the openings of the cooling compartment, the cooling apparatus comprising:
at least one plane rectifier plate disposed above the openings of the cooling compartment in parallel with the circuit board which is conveyed over the open top of the cooling compartment, having a plurality of holes, and adapted to rectify the cooling air;
a plurality of inclined rectifier plates disposed on the plane rectifier plate in a plurality of lines in a direction of conveying the circuit board, inclined at a given angle in the conveying direction, and adapted to rectify the cooling air from the plane rectifier plate; and
a plurality of air guide plates made of a heat-resistant flexible material, disposed toward distal ends of the inclined rectifier plates at intervals in the conveying direction, and allowing their distal ends to reach the circuit board.
According to a second aspect of the present invention, in the structure of the first aspect of the present invention, a length for which the air guide plate is in contact with a soldering face of the circuit board is set to be 3 cm or smaller but larger than 0 cm.
According to a third aspect of the present invention, in the structure of the first or second aspect of the present invention, the air guide plates are made of polyimide.
According to a fourth aspect of the present invention, in the structure of the first or second aspect of the present invention, the air guide plates are made of glass cloth impregnated with fluorine resin.
According to a fifth aspect of the present invention, in the structure of any one of the first to fourth aspects of the present invention, the air guide plates are coated with an antistatic material.
According to a sixth aspect of the present invention, in the structure of any one of the first to fifth aspects of the present invention, the air guide plates are detachably attached to the inclined rectifier plates.
According to a seventh aspect of the present invention, in the structure of any one of the first to sixth aspects of the present invention, the cooling compartment is sectioned into a plurality of zones by at least one partition, each of the plurality of zones has each of the openings through which the cooling air passes, and a valve is disposed on each of branch pipes of a blast pipe, which communicate with the respective openings.
According to an eighth aspect of the present invention, in the structure of any one of the first to seventh aspects of the present invention, an inclination angle of the inclined rectifier plates is set to 30 to 70xc2x0.